High-performance, large-area integrated circuits (IC's) often require several levels of interconnect. Planarization processes which smooth and flatten the surface of an IC at various stages of fabrication are becoming essential, both for high-resolution photolithography and for adequate coverage of steps by thin films. Numerous techniques exist for planarizing the insulating layers in multilevel IC's, e.g., spin-on insulators, spin-on sacrificial layers followed by nonselective etching, and RF bias sputter deposition. CO.sub.2 lasers have been used in both CW and pulsed operation to rapidly flow phosphosilicate glass over aluminum interconnects.
Planarization of the insulating layers does not by itself yield a fully planar multilevel interconnect process. Severe step-coverage problems still occur where metals are deposited over deep vertical vias in an insulator. The problem is magnified if vias are vertically stacked. These difficulties are typically mitigated by tapering the via walls (which consumes valuable area), limiting their depth/width ratio, and forbidding stacked vias. Alternatively, selective deposition techniques (tungsten chemical vapor deposition or lift-off or metal pillar fabrication) show promise for filling deep vias. Yet another solution is to planarize each metal film prior to patterning. Metal planarization could be combined with insulator planarization to yield a fully planar IC process; it might also be used by itself in specialized multilevel interconnect structures where power and ground planes are interspersed between signal levels (e.g., "silicon PC boards" for advanced IC packaging). One metal planarization technique is RF bias sputtering.
One impediment to planarization of aluminum layers by RF bias sputtering is the presence of oxygen in the system which combines with the aluminum to form aluminum oxide. The melting point of aluminum oxide is much higher than that of aluminum and an aluminum film contaminated with aluminum oxide resists planarization.
Magnetron sputter devices are characterized by crossed electric and magnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced. The gas is ionized by electrons accelerated by the electric field. The magnetic field confines the ionized gas, which forms a plasma in proximity to a target structure. The gas ions strike the target structure, causing emission of atoms that are incident on a workpiece, typically a substrate in a coating process. Generally, the magnetic field is established by a permanent magnet structure, although electromagnetic devices are increasingly being employed for this purpose. In coating applications, the magnetron sputtering devices are frequently employed to deposit metals in the manufacture of electronic integrated circuit type devices. It is also known to deposit magnetic materials in the manufacture of high density magnetic discs of a type used for magnetic disc memories.
In prior art magnetron sputtering devices, uniform coating thickness across a substrate was obtained by moving the substrates during coating. Moving the substrates also assisted in obtaining step coverage, i.e., conformal coating over step-type transitions. Of course, there are many problems in moving a substrate during operation of a sputtering device. It is also desirable in certain instances to co-deposit different materials, particularly materials which are difficult or impossible to alloy; that is, materials which are not adapted to being on a single target. In all instances, it is desirable to operate the sputtering device at as high a rate as possible.
Sputter sources incorporating only permanent magnets, the typical prior art arrangement, do not enable the plasma confining magnetic field to change over the life of the target. In consequence, the impedance of the sputter device, i.e., the ratio of the discharge voltage which establishes the electric field to the discharge current flowing in the plasma, decreases steadily as the target erodes during use. The power supplies necessary to provide the electric field are therefore relatively complicated and expensive in an attempt to match the varying sputter device impedance over the target life.
As the target surface erodes during use, the target has a tendency to create a shadow for material emitted from the source. Thereby, the gross efficiency of the sputter device decreases as the target erodes during use. Because of the shadowing effect, the rate at which material is deposited on a substrate decreases usually in a non-linear manner, as the target erodes.
One attempt to minimize the reduced deposition rate caused by the shadow effect involves revolving an assembly including the permanent magnet about an axis of the sputtering device. Revolving the magnet assembly results in a substantial improvement in the efficiency of the sputtering process near the end of target life, but a decrease in the impedance of the device still has been observed as the target erodes. In addition, the rate at which material is sputtered from the target also decreases as the target erodes with this approach. Of course, rotating the permanent magnet structure is mechanically complex.
While many of the problems associated with the permanent magnet arrangement have been obviated by using electromagnets, the electromagnet devices have generally had the disadvantage of using single targets, having relatively narrow widths of approximately one inch. There has been recently developed systems wherein the targets have been configured as assemblies having plural target elements, generally concentric with each other. In one configuration, the targets are both planar elements; in a second configuration, an inner target is planar and an outer target is concave, having an emitting surface defined by a side wall of a frustum of a cone. These prior art devices are effective to enable material to be deposited uniformly over a large area workpiece, such as a substrate being coated.
It has been observed that the relative contributions of the two targets on the workpiece change differentially as the targets erode during use. In other words, the amount of material reaching the workpiece from the first target changes relative to the amount of material reaching the workpiece from the second target as the targets are being consumed or eroded. Thus, designing a controller for multiple element target assemblies to achieve uniform impact of material on the workpiece during the useful life of the target assemblies is complex, and not straightforward. This is particularly the case for uniform deposition across relatively large area workpieces, such as a six-inch integrated circuit wafer or a hard computer storage magnetic disc. The system is also complex because of the need and desire to control the impedances of the plasma discharges during the changing conditions that occur as the targets erode.